how to use displacement node in 2.8
The Normal input socket of the displacement node is meant to be used for further normal maps that are applied on top of the displacement (daisy chaining multiple maps, just like you can do with bump maps). For connecting the displacement texture itself, the Height socket has to be used. Scale can then be used to multiply the result, i.e. modify the strength. As usual, these are Blender units.
The main input sockets are described in the manual. So to summarize, the node connection has to be like this:

The rest is just like you had it in your example. This is a brick texture displacing the default cube:

One thing to mention, as you did not explicitly state it: If you want to go for Adaptive Displacement (sometimes called Micro-Displacement), you need to use the Experimental feature set, and adjust the Dicing Rate to your needs:

The SubSurf modifier of the object also needs to be changed a little. Adaptive Subdivision has to be checked:

The example rendering above uses adaptive displacement.